1. Field of the Invention
The present invention relates to a method for controlling a lighting device, in particular a lighting device that is meant to be swung around when in use, for moving the projected image on a screen, wall or such.
2. Discussion of the Related Art
Lighting devices are known comprising a light source and a mechanical light valve consisting of a rotatable wheel containing several so called gobo's that can be placed in front of the light source.
A gobo is hereby generally defined as a plate of metal or the like, arranged with a number of holes in it or with a figure curved out of it.
The gobo placed in front of the light source hereby blocks part of the light beam emitted by the light source, while the rest of the light beam passes through the holes or the curved out figure, and is projected to a screen forming an image in the shape of said holes or figure.
In order to change the image projected by the above described lighting device, one can rotate the wheel containing the gobo's until another gobo is situated in front of the lighting source.
An inconvenience of the present device and method is that the number of gobo's that can be provided in the wheel is very limited—usually about twelve—thereby limiting the visual effects that can be attained by said known lighting device.
In order to overcome said inconvenience, it is known to replace the mechanical light valve by a digital light valve, comprising an array of light processing elements. A common used example of such digital light valve is a digital micro-mirrors device or DMD that is generally known as an array of individual micro-mirrors reflecting the light beam emitted by the light source on a lens where through it is projected on the screen, whereby each mirror corresponds to a pixel of the image to be built and can be switched individually between a position wherein the beam is reflected towards the lens and a position wherein the light beam is reflected away from the lens. So when a mirror is positioned in an on-screen position, the corresponding pixel on the screen is lit up, whereas when the mirror is positioned in an off-screen position, the corresponding pixel remains unlit.
Since the position of every mirror can be controlled individually by a programmable stream of bits, the use of the DMD allows a nearly unlimited combination of images to be created and projected.
In a specific known embodiment, a color sequential device is provided in between the light source and the digital light valve, such color sequential device consisting of a color disc comprising a red, a green and a blue filter section, or rotating prisms.
When said color sequential device is rotated in front of the lighting source, the light beam emitted by this last will subsequently change of color before being processed by the light valve.
Since the shifting of the colors appears faster than the time of the human perception of vision, the human eye sees a full colored image, being the average of the subsequently projected color beams.
Indeed, each pixel of the image projected on the screen is composed of three subsequently reflected color beams, whereby each color beam is projected for a certain time. During this time each mirror can shift several times between an on-screen and an off-screen position, so that only part of the color beam used for one color of a pixel reaches the screen.
The more time a specific mirror is positioned in an on-screen position, the brighter the color will appear on the corresponding pixel.
The shifting of the mirrors is hereby controlled by a sequence of bits, each bit having two possible values, namely 0 and 1 each corresponding to respectively the off-screen and the on-screen position of a mirror. The number of bits present in a bit sequence used to control a mirror while reflecting a beam of a specific color is hereby proportional to the times that the mirror can shift during the time one single color of the image is projected. It is clear that the more bits used to program the shifting, the more variations in color can be reached.
Said number of bits used to control the position of a single mirror during the processing of one single color of an image is generally known as the bit depth.
Presently it is common in the art of projecting images to use a bit depth of 16 to create nice looking colored images.
Another important quality parameter in the projection of moving images is the refresh rate, which is inversely proportional to the time it takes to project an entire image on the screen, more exactly the sum of the times each of the color filters red, green and blue are situated subsequently in front of the light source.
The refresh rate should hereby be faster than the human perception of vision, since otherwise the moving image will not be experienced as moving in a smooth continuous manner.
The refresh time of an image now seems to be the most important drawback of the known lighting devices comprising a digital light valve.
Indeed, in the above described lighting devices with a bit depth of 16, the refresh rate is about twice the time of human perception of vision, which is not enough when the lighting device is swung around, since in that case a color break up occurs that is due to the fact that the different colors of an image do not perfectly overlay anymore.